Darwin defined certain qualities that organisms needed to possess in order to
enable the evolution process: Inheritable mutational changes to organism
designs had to occur. Natural variation in designs among members of a
species had to consequently exist. Organisms had to be in competition for
resources. Natural selection therefore operated on all populations of all
species.

Darwin had no reason to believe that all these qualities were not fundamental
qualities that applied, equally, to all living organisms. All species were
subject to mutations, competitive forces, and natural selection. All organisms
therefore possessed the same capacity for evolution. Evolutionary capacity or
evolvability was a constant that did not need to be specifically handled by an
evolutionary mechanics theory. Traditional evolutionary mechanics still takes
this position.

However, genetics science
eventually produced a long list of discoveries that cast doubt on the idea of
constant evolvability in a number of different ways. Recall that biological
inheritance is critical to evolutionary mechanics. Any mutational change has to
be inherited myriad times in order to propagate to a population, much less an
entire species. If inheritance mechanisms handled all mutational changes in
exactly the same way, inheritance could be disregarded as a variable factor in the
evolutionary process. Genetics discoveries have definitively determined that
this is not in fact the case. For example, a mutational change to an X or Y
chromosome or to mitochondrial DNA propagates in an entirely different way from
a change to the other chromosomes. Some organisms (e.g. bacteria) only possess
one haploid set of genetic data. More complex diploid organisms possess two
sets of genetic data that jointly specify organism design. Mutational changes
propagate in very different ways in these two cases. A mutation to
genetic data in a diploid organism might have essentially no phenotypic effect unless
both sets of genetic data possessed the mutation. This tends to retard
propagation of beneficial changes and enhance propagation of adverse changes
relative to the haploid case. Another example: some
genes are essentially duplicated such that a mutational change to one would have
proportionally less effect than a similar change to some other gene that was
less redundant, thus changing the way such a mutation would propagate and
introducing robustness or resistance to mutational change. The above are only a few of
the many instances of genetics discoveries that cast doubt on the idea that
evolvability is a constant or otherwise present issues for traditional
evolutionary mechanics theory. See How
Genetics Discoveries Affect Evolution Theory for more information.

Theorists suggest that not only
does evolvability vary between organisms, but that it is a selectable property.
Organisms can acquire through the evolution process increases in their
evolvability just as they can acquire increases in fitness. Most of the
evolvability possessed by complex organisms may result from evolved
characteristics such as sexual reproduction and design-limited life span.
Organism characteristics that increase evolvability tend to decrease individual fitness.
Therefore, fitness is a trade-off with evolvability. There are evolvability
explanations for all of the observed
conflicts with traditional evolutionary mechanics.

Some followers of traditional
evolutionary mechanics theory have criticized evolvability theories as simply
another form of group selection. However, the mechanics of evolvability
propagation are logically rather different and valid criticism would need to
deal with the differences.

A number of evolvability
advantages have been proposed for a design-limited life span including the
following:

Increases genetic diversity by
decreasing potential for a relatively few older individuals to dominate the gene pool.

Increases the adult death rate.
Adult death rate is the equivalent to adult lives lived per unit time. Increases
the number of organisms participating in the evolution process.

Allows evolution of intelligence,
immunity, and other organism design characteristics whose utility depends on
acquisition of something that non-genetically adds to fitness.

Some theorists suggest that
gradual aging seen in most mammals and larger animals has additional
evolvability benefit over biological suicide: Gradual deterioration provides a
challenge that can be overcome by fitness thereby increasing the apparent
differential between more fit and less fit organisms and thus increasing
evolution rate.

Many organism characteristics
appear to have evolvability implications including age of sexual maturity and
other restrictions on reproduction such as some mating rituals and behaviors. An
evolvability-based programmed-aging version of the Maintenance Theory of Aging
suggests a special relationship between aging and sexual maturity age.